Keychain Access version 3.0 (in /Applications -> Utilities) that ships with Jaguar has a feature that allows you to store text in a Secure Note. This is really useful if you have free form data that you want to keep on your computer with strong encryption. There are dozens of great uses for this including credit card or bank account information.
Mac OS X Hints
http://hints.macworld.com/article.php?story=20020911065901293